It is often desired to generate a voltage of a particular value, such as for use within a circuit. A converter may be used for this purpose, receiving a Direct Current (DC) input voltage, and outputting a DC voltage at the desired level. The input DC voltage, however, sometimes changes value, and it becomes necessary to adjust the converter to maintain the output voltage substantially constant.
Traditionally a converter was made from a voltage regulator that uses feedback. In other words, the output voltage is sensed (“fed back”), and then used to control the regulator. In a particular type of application, the regulator is a switching regulator, and control is by modulating the width of pulses (“pulse width modulation”, or “PWM”).
Another technique that has proven very useful in voltage mode pwm controllers is feedforward, which senses the input voltage in addition to the output voltage. The pwm ramp is modified according to the sensed input voltage—for example a peak to peak voltage is increased as the input voltage increases. A disadvantage of the feedforward topology is the required sensing of the input voltage. Another disadvantage is that the pwm ramp becomes vulnerable to noise on the input voltage, which requires some filtering to overcome.
In a voltage mode architecture, the cascade connection of the pwm modulator and the control to output transfer function are sometimes referred to as the plant (the object to be compensated). In some instances the gain of the plant is directly proportional to the input voltage. A problem has been when it is desired to compensate the transfer function to have enough phase margin for the highest input voltage case, because that causes the performance to suffer at the lowest input voltage case. The feedforward technique has solved the problem by having an adjustable modulator gain inversely proportional to the input voltage, thus rendering the compensation of the transfer function independent from the input voltage. Nevertheless, that is a complex solution.